Pulse tube refrigerator

ABSTRACT

A pulse tube refrigerator ( 1 ) comprises one or more stages ( 7, 10 ). Each stage comprises a pulse tube ( 5, 8 ) and a regenerative heat exchanger ( 6, 9 ), the heat exchanger comprising a foam matrix material ( 16 ).

This invention relates to a pulse tube refrigerator (PTR).

For the purpose of this application, a regenerative heat exchanger is one in which a fluid flows through a material which stores heat and when the same fluid flows through the material subsequently, then the stored heat is given up from the material to the fluid flow. In operation of a pulse tube refrigerator, this occurs periodically. By contrast, a recuperative heat exchanger is one in which at least two different fluids are provided which may flow simultaneously and in particular do not flow through the same material.

A recuperative heat exchanger is not suitable for use in a pulse tube refrigerator because only a single gas component (e.g. He, Ne, or other gases) is used when operating the PTR, although recuperative heat exchangers are commonly used at the warm end and cold end of the pulse tubes of the PTR where they provide additional cooling, for example, a water cooled heat exchanger.

Another common use of the term regenerative relates to situations in which the material within the heat exchanger can be regenerated for further use if it becomes saturated. However, this type of material is also not appropriate for use in a pulse tube refrigerator because the PTR operates with a clean gas, so the matrix does not become saturated by other components, nor does the efficiency of the regenerator become reduced.

Pulse tube refrigerators (PTR) use regenerative heat exchangers to increase or decrease the temperature of a cryogenic liquid, typically Helium gas. Regenerator effectiveness is critical to performance of the PTR during the PTR operation. A matrix is provided in the regenerator through which Helium gas flows to lose or gain heat. The gas travels in a first flow direction through the regenerator matrix and in the process it gives off heat and thereby experiences a decrease in temperature. The specific heat capacity of the matrix material enables the heat extracted from the Helium to be stored in the matrix material and subsequently, when the gas travels back through the matrix in the reverse direction, it takes stored heat from the matrix and so experiences a temperature rise. To achieve high effectiveness, the regenerator matrix material has to satisfy stringent thermal performance requirements. These requirements are that the regenerator matrix materials have a high heat capacity compared to the gas heat capacity; that there is at least a minimum pressure drop, so requiring a high porosity; and that there is a high heat transfer area, usually achieved by means of a divided matrix in the form of mesh.

Conventionally, a metal mesh is used as the matrix material for a single stage regenerator operating between 300K and 25K or for the first stage of a two stage regenerator for a 10K or 4K PTR. Typically, the metal used to make the mesh is copper, stainless steel or phosphor bronze. However, fabrication of the meshes is a major task which involves making a die, punching and stacking the meshes manually in the regenerator tube and then aligning the meshes in the tube. The cost and time involved in this is quite high.

A second stage regenerator of a 10K PTR uses Pb as the regenerator matrix material and a 4K PTR typically uses Pb, ErNi , HCu₂, or Gadolinium based compounds, depending upon the desired temperature of operation, as regenerator matrix materials, but these materials cannot be made up into a mesh, so they are used in the form of spherical balls instead. The use of spherical balls means that the porosity of the resulting matrix is only of the order of 25% to 35%. A problem with this is that all the spherical balls must have the same diameter and must be packed in a particular way to achieve the desired porosity. As the ball diameter can vary about the nominal diameter and the packing may not be optimum, then it is not possible to guarantee that the desired porosity is actually achieved. This can have a drastic effect on performance.

In accordance with a first aspect of the present invention, a pulse tube refrigerator comprises one or more stages, wherein each stage comprises a pulse tube and a regenerative heat exchanger, the heat exchanger comprising a foam matrix material.

The present invention makes use of a foam matrix material to replace the mesh in the first stage regenerator of the PTR, thereby simplifying regenerator assembly and reducing costs and uses a foam matrix material to replace the spherical balls in the second stage regenerator, so improving the porosity. This latter means that any porosity can be chosen and it can be accurately controlled so improving performance of the PTR.

Preferably, the foam is of metal or ceramics. For the first stage of a 10K PTR, the foam is generally a metal foam. For the second stage, it may be a metal foam, or a foam based on ceramic compounds, or a combination of metal and ceramic compounds. For a PTR working at 4K and below, preferably, for the second stage regenerator, the ceramic compounds include Al₂O₃, GdAlO₃, aluminium nitride AlN-type ceramics, or further Gadolinium based compounds.

More preferably, the foam is one of copper, copper bronze, stainless steel, lead, erbium nickel or holmium copper foam.

The selection of the material type and quantity required in the regenerator stages of a PTR depends upon the targeted stage temperatures for any particular application. These materials are known to have suitable heat capacity properties, although other materials with similar properties could be used.

Porosity of the foam can be adapted as required for the particular application, for example for the first stage, preferably, the foam has a porosity in the range 75% to 95% and for the second stage, preferably, the foam has a porosity in the range 25% to 40%.

In accordance with a second aspect of the present invention, a pulse tube refrigerator comprises one or more stages, each stage comprising a pulse tube and a regenerative heat exchanger according to the first aspect.

Preferably, the pulse tube refrigerator further comprises at least one of warm end and cold end recuperative heat exchangers, wherein the recuperative heat exchangers comprise a foam matrix material.

Typically, the foam matrix material for the recuperative heat exchangers comprises copper, copper bronze or other copper alloy type foam.

Typically, the regenerative heat exchanger operates in a temperature range of 2K to 10K.

An example of a regenerative heat exchanger for a pulse tube refrigerator in accordance with the present invention will now be described with reference to the accompanying drawings in which:

FIG. 1 illustrates a two stage pulse tube refrigerator, each stage incorporating a regenerative heat exchanger in accordance with the present invention; and,

FIG. 2 shows the construction of the regenerative heat exchanger in one stage of FIG. 1 in more detail.

FIG. 1 shows a PTR system configuration 1 incorporating a regenerative heat exchanger (regenerator) according to the present invention. Refrigerant gas, typically Helium, is supplied from a compressor through a valve system 3 which distributes the gas into a cold head comprising a high (room) temperature end 4, a first pulse tube 5 and first regenerator 6 connected between the high temperature end 4 and a first stage 7, and a second pulse tube 8 and a second regenerator 9 connected between the high temperature end 4 and a second stage 10. Each pulse tube 5, 8 is hollow and used for expansion and compression of the gas. Warm end 11 and cold end 12 recuperative heat exchangers are provided at respective ends of each pulse tube 5, 8. Typically, these heat exchangers 11, 12 are made of metal meshes which are vacuum brazed or press fit into a copper casing across which heat transfer takes place. However, in a pulse tube refrigerator in accordance with the invention, these heat exchangers may also be provided with metal foams instead, giving further benefits in terms of manufacture and operation. Gas flow in the cold head is ac flow, in that it flows in and out through the same flow passages. Operation of the PTR produces cooling of the stages, in this case the first 7 and second 10 for a two stage refrigerator. The gas supply then returns to the compressor 1.

Each regenerator 6, 9 is filled with a foam 13, 14 which acts as a heat buffer to enable exchange of heat with the Helium gas of the PTR. The type of foam depends upon the temperature range of operation of the regenerator. In the first stage regenerator 6, in which conventionally the matrix is made from a metal mesh, copper or stainless steel foams are suitable for use as regenerator matrix materials, as the heat capacity of both materials is acceptable in the temperature range of 300-30 K. Furthermore, the porosity of a conventional metal mesh has a maximum of 75%, whereas the porosity of foam can be increased up to 95% and this will help in reducing the pressure drop of Helium gas which is quite critical from the PTR performance point of view

In the second stage regenerator 9, lead foam matrix material may be used for a 10K PTR, or a foam of compounds of ErNi for a 4K PTR. In the second stage the improvement in porosity using foams of materials with the same heat capacity as in a conventional regenerator matrix, is even more significant, increasing from the 25% to 35% porosity achieved with spherical balls.

FIG. 2 shows one of the regenerators of FIG. I in more detail. The regenerator 6 comprises a stainless steel tube 15 and a matrix, in the form of a metal foam cylinder 16, inserted into the tube to replace the conventional metal mesh. In conventional regenerative heat exchangers for a PTR, it may be necessary to produce as many as 1500 pieces of mesh of a chosen porosity and form the matrix from these, whereas using a foam matrix material has the advantage that the regenerator matrix material can be manufactured in the finished cylindrical form and put into the regenerator tube directly, or brazed if necessary, so significantly reducing the time and cost of manufacturing. The present invention makes use of the properties that foams are very light materials, with a high mechanical strength. They are able to operate at the extremely low temperatures required for PTR'S, which had previously been considered impractical due to the dramatic changes in the basic material properties expected at the operating temperatures of the high pressure Helium gas flowing through the matrix. Foams have got high porosity and high heat transfer area per unit volume, so they are advantageous both from functional as well as fabrication point of view. 

1. A pulse tube refrigerator comprising one or more stages, wherein each stage comprises a pulse tube and a regenerative heat exchanger, the heat exchanger comprising a foam matrix material.
 2. A pulse tube refrigerator according to claim 1, wherein the foam comprises metal or ceramics.
 3. A pulse tube refrigerator according to claim 1, wherein the foam comprises one of copper, copper bronze, stainless steel, lead, erbium nickel or holmium copper foam.
 4. A pulse tube refrigerator according to claim 1, the foam comprising a metal foam including ceramic compounds, wherein the ceramic compounds include Al₂O₃ , GdAlO₃, AIN-type ceramics, or further Gadolinium based compounds.
 5. A pulse tube refrigerator according to claim 1, wherein the foam has a porosity in the range 75% to 95%.
 6. A pulse tube refrigerator according to claim 1, further comprising at least one of warm end and cold end recuperative heat exchangers, wherein the recuperative heat exchangers comprise a foam matrix material.
 7. A pulse tube refrigerator according to claim 1, wherein the regenerative heat exchanger operates in a temperature range of 2K to 10K. 